In a search for agents with clinically used carcinostatic activity, we would like to continue our work on the synthesis of antagonists of folic and pteroic acid. Work is to be continued on the synthesis of lipid soluble derivatives of folic acid antagonists, the 7-isomers of pteroic and folic acid, analogs in which the N10 nitrogen is replaced with sulfur, 4 thio analogs of folic and pteroic acid, as well as studies of the biological activities of the compounds to be synthesized. In particular, ability to inhibit suitable microbial systems, dihydrofolic acid reductase, and thymidylate synthetase, as well as ability to inhibit the growth of mouse tumors are to be explored. The synthesis and study of flourescent probe molecules for obtaining detailed information about the interactions of biologically active pteridines with their target enzymes is outlined, as are studies of the conformation and the factors involved in maintaining the conformation of folic acid, homofolic acid, and related compounds.